Lactation in the rat is characterized by an inhibition of ovarian cyclicity which correlates with a suppression of gonadotropin secretion. The proposed studies are designed to determine how the suckling stimulus and the elevated levels of prolactin associated with lactation suppress pituitary and hypothalamic function. To delineate the mechanisms involved in the suppression of pituitary function, three aspects of pituitary function will be assessed in vitro: 1) responsiveness to LHRH, 2) the rate of LH synthesis and 3) the rate of LH release. Further studies are designed to differentiate the effects that the suckling stimulus has on these aspects of pituitary function (presumably reflected as a decrease in LHRH release from the hypothamalus) from the effects due to prolactin. Studies to delineate the effects of lactation on hypothalamic function will focus on the role that the increase in dopamine activity associated with lactation plays in the suppression of LHRH activity and gonadotropin secretion. The relative contribution of suckling and prolactin to the suppression of gonadotropin secretion will be determined following ovariectomy and electrical stimulation of the median eminence-arcuate nucleus areas. The effects of stimulating or blocking dopamine receptors on gonadotropin secretion will be assessed in conjunction with changes in the intensity of the suckling stimulus or prolactin concentrations.